Green HVAC Designs for Data Centers

Because of their computer and HVAC equipment, data-processing centers are among the largest commercial users of electrical energy. While the energy consumed by data-processing equipment is fairly constant year-round, considerable energy savings are possible with an innovative approach to data-center HVAC design.

The entire electrical-kilowatt-power input to computer equipment is converted to heat and discharged into a conditioned space. Most small and medium-size data-processing centers utilize direct-expansion computer-room air conditioners — typically, split-air, water, or glycol-cooled. Large data centers predominantly utilize chilled-water-room units for several reasons, including:

Increased efficiency of central chillers.

Elimination of excessive refrigeration piping and charging.

Simplified maintenance of the central refrigeration plant.

Greatly reduced outside-equipment footprint.

Per American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standards, typical data-center HVAC design conditions (68 to 77°F, 40- to 45-percent relative humidity) are required day and night, 365 days a year. Chilled-water temperature usually is required to be 45 to 50°F, depending on the computer-room-air-conditioning- (CRAC-) unit coil design for cooling and dehumidification. This continuous temperature- and humidity-control requirement also allows the possibility of substantial energy savings.

Two significant energy-saving possibilities for chilled-water systems are the use of free-cooling chillers (in central and northern states and Canada) and simultaneous heat-recovery chillers for dehumidification (in southern states).

FREE COOLING

The continuous cooling demand of data centers is largely unchanged during winter because winter building losses do not have a significant effect on total heat load. With the application of a free-cooling circuit to a central chiller, refrigeration compressors can be switched off for long periods of time during winter, spring, and fall. A water-cooled central chiller can be designed to switch automatically to free cooling as soon as a cooling tower's cooling water or glycol is at or below the return chilled-water temperature, while an air-cooled chiller with an integrated or separate free-cooling coil can switch to free cooling when the outside ambient temperature is lower than the return chilled-water temperature. Both of these energy-efficient designs for large data-center HVAC systems have similar advantages, including a direct reduction in energy costs and fewer compressor running hours. Further, the refrigeration compressors in an air-cooled chiller are never required to operate in very cold ambient temperatures, which eliminates any special low-temperature refrigeration controls and extends the equipment's life expectancy by eliminating the most difficult operating conditions for an air-cooled system.

Consider the savings possible in a New York City data center with 20 25-ton chilled-water CRAC units. This represents an installed cooling capacity of 500 tons. ASHRAE's historical weather data in New York City suggests a 99-percent winter design dry-bulb temperature of 11°F and an average of 3,750 hr of dry-bulb temperatures below 40°F per year. The chilled-water CRAC units typically require approximately 50°F chilled-water-supply and 60°F chilled-water-return temperatures. This means an ideal situation exists for winter free-cooling chiller savings.

Water-cooled chillers

A water-cooled chiller with an economizer is an energy-saving design solution. Winter energy savings from this system design are possible using cooling-tower water whenever it is below 60°F. An open cooling-tower winter economizer loop for chilled-water computer-room units would require an intermediate plate-and-frame heat exchanger to keep the data center's chilled water in a closed loop and free from external contamination. The tower must be winterized, via a variable-frequency drive or fan cycling, with heating and piping thermal insulation. Maintenance would include the chemical treatment of the makeup water, routine blowdown and cleaning, and fill replacement as required.

For redundancy in a critical data center, a duplicate parallel tower should be installed, along with a parallel plate exchanger and adequate filtration of tower water. These items, as well as the addition of piping for winter crossover operation, create a thermally efficient, but relatively expensive and complicated, piping system to provide required redundancy and winter energy savings.

If two 250-ton water-cooled chillers were installed in the New York City data center — each with compressor power of 188 kw, or 376 kw total (assuming two R-407C screw compressors per chiller) — the annual winter free-cooling savings would be:

The total savings would be about $68,402 more per year (conservatively rounded into 25-, 50-, and 100-percent free-cooling temperature bins) than they would with identical water-cooled chillers without free cooling.

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With condenser-inlet-water temperatures of 60°F (prior to free cooling), water-regulating valves would maintain a minimum acceptable head pressure of approximately 240 psig (110°F). In typical summer conditions, when tower water is 85°F, water-cooled chillers will have slightly lower operating costs than air-cooled chillers operating in 95°F ambient temperatures. However, these savings are offset by the added expenses of cooling-tower fans and pumps.

A closed-loop evaporative tower (with a backup tower) offers a cleaner solution for water-cooled chillers because the circulated process fluid is a glycol solution that flows in a completely closed loop. Spray-water treatment still is required, as are a sump heater and winter fan controls. A plate exchanger installed similarly to an open tower can provide partial or 100-percent free cooling. The closed-loop glycol solution, however, also can be diverted automatically from the chillers' water-cooled condenser to the room units without the need for an intermediate exchanger whenever conditions are right for 100-percent free cooling. Although a closed-loop system is simpler than an open-loop system and eliminates plate-and-frame exchangers, it excludes the possibility of partial free cooling. Also, the initial cost of a closed-loop evaporative tower is significantly higher than that of an open-draft tower.

In both closed- and open-loop systems, water-cooled chillers can operate in full mechanical mode until tower water is cold enough to satisfy the chilled water required by room units. They also can operate in partial-free-cooling mode if an intermediate exchanger is employed. Tower water needs to be approximately 40 to 45°F — assuming a 5- to 10°F approach in a plate-and-frame exchanger — before 100-percent free cooling is available and 50°F chilled water can be supplied to room units without compressor operation.

Air-cooled chillers

An air-cooled chiller with integrated free cooling provides energy savings whenever ambient dry-bulb temperature is below 60°F. If two 250-ton air-cooled free-cooling chillers were installed in the same 500-ton application, the absorbed compressor power (assuming the same two R-407C screw compressors per chiller) would be approximately 188 kw per chiller, or 376 kw total, in 60°F outside ambient temperature, delivering 50°F chilled water/glycol to the room units. The fan controls would limit winter head pressure to a minimum of approximately 240 psig (110°F) to maintain suction pressure and effective compressor lubrication. A 50-percent free-cooling design is practical for integrated free-cooling coils when the ambient temperature is 10°F below the required chilled-water temperature, while a 100-percent free-cooling design is practical when the ambient temperature is 20°F below the received chilled-water temperature.

Using temperature-bin data for New York City, estimated power savings for our example data center with air-cooled free-cooling chillers would be virtually identical to that of the water-cooled chillers. Again, the total savings would equal about $68,402 more per year (conservatively rounded into 25-, 50-, and 100-percent free-cooling temperature bins) than identical winterized air-cooled chillers without free cooling. Redundancy design considerations likely would require a third parallel redundant chiller for data-center operation.

Utilizing an outside packaged air-cooled chiller with dry coolers (radiators) installed in series in a chiller's return line also can save a considerable amount of free-cooling energy. This design can be used as a practical retrofit for any existing air- or water-cooled-chiller installation. A three-way diverting valve directs return chilled water/glycol through dry coolers whenever the outside ambient temperature is cold enough for partial free cooling. The dry coolers' fans are enabled automatically, with fan cycling or variable-frequency-drive fan controls minimizing fan power while also preventing overcooling of the chilled water/glycol amid extreme winter temperatures. The three-way valve can be used to partially bypass the dry coolers if necessary.

This system can be designed to achieve an even closer approach to winter ambient temperatures to maximize energy savings. However, the tradeoff requires significantly increased dry-cooler surface, fan power, and outdoor footprint because the low initial temperature differential and close approach temperature require an exponential increase in surface area.

Because the chilled glycol in an integrated free-cooling chiller is in a completely closed circuit, there is no requirement for a plate heat exchanger, crossover piping, or valves. No water treatment, evaporation, or water makeup is required. A three-way valve is incorporated inside the chiller to initiate and modulate free cooling, and a programmable logic controller automatically controls all of the system's operating and alarm functions.

Packaged air-cooled free-cooling chillers typically are available up to 300 tons. They also usually are available with an integrated chilled-glycol storage tank and one or two pumps. It is highly recommended to split the cooling load into two or more chillers for data-center operation so that redundant capacity always is available in the event of maintenance or an equipment malfunction.

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Packaged chillers from 70 to 300 tons are available with dual screw compressors in independent refrigeration circuits to provide added reliability and redundancy. Step or stepless unloading compressor controls allow for changing data-center load conditions and can closely match the power input to an applied load when compressor operation is required.

Chillers above 300 to 350 tons typically have a centrifugal design with a single compressor. They generally are only water-cooled and can be utilized with a cooling-tower free-cooling system as described previously.

HEAT-RECOVERY CHILLERS

While free cooling is possible in warmer climates, savings are limited by the need for ambient-air or cooling-water temperature to be lower than return chilled-water temperature. This reduces the number of available free-cooling hours, but a careful study of temperature-bin data can reveal possible savings at any location. For data-center HVAC systems in the South and West, however, the demand for a dehumidification cycle (simultaneous cooling and reheating) provides yet another opportunity to save on electrical demand charges and continuous electrical energy.

While the use of reheat is not recommended because of its waste of energy, it is utilized in many southern locations because of the influence of high-humidity makeup air and/or vapor-pressure equalization when the vapor barrier around a data center is not secure. Computer-room units in these installations can be specified with hot-water-reheat coils (in lieu of electric-resistance heaters), and the chillers can be specified with up to 90-percent continuous (series) heat recovery to a hot-water loop at 90 to 110°F for reheating purposes.

This is preferred to parallel heat recovery, which typically is available as “all or nothing” heat. Series heat recovery means hot discharge refrigerant gas always passes through a heat-recovery exchanger before entering a condenser. No additional refrigeration controls are required, and the maximum available heat always is supplied to the hot-water loop. Therefore, sufficient (free) reheat always is available to compensate for any cooling required during the dehumidification cycle because the total available condenser rejected heat is approximately 130 percent of the evaporator's capacity. Greater than 90-percent recovery of 130-percent evaporator capacity equals reheat available at greater than 117 percent of total cooling capacity. Heat recovery is available from air- or water-cooled chillers, and most existing chillers can be retrofitted.

Consider the savings on any large CRAC installation: The same New York City example data center located in Atlanta, with 20 25-ton chilled-water CRAC units, requires 500 tons of cooling capacity. If reheat were required, each of these room units would require approximately 22.5 kw of electrical-resistance heat for dehumidification reheat. This would represent 562 kw of installed electrical heat, which would be added to the electrical demand charge of the building.

If dehumidification were required for about 20 percent of the year at a cost of 8 cents per kilowatt-hour, the annual power cost would be 450 kw times 24 hr times 365 days times 0.2 (20-percent assumed dehumidification) times 8 cents per kilowatt-hour, which equals $63,072. In addition, the demand charge would be approximately $10,000 per year, for a total annual savings of $73,072.

The cost of installing low-pressure hot-water heating coils with control valves in CRAC units is a little higher than the cost of installing electrical-resistance-heater coils and associated controls, but the continuous demand and energy savings far outweigh the initial cost. It should be noted that specifying excessively high water temperatures for reheat should be avoided because chiller efficiency is reduced by high condensing temperatures.

CONCLUSION

Large data centers represent an opportunity for major HVAC energy savings by utilizing chillers with integrated winter free cooling or heat-recovery chillers for free reheat during dehumidification. In either case, the payback period typically will be less than one year. All of these systems can help a project earn Leadership in Energy and Environmental Design credits.

The president of Motivair Corp., Graham Whitmore has specialized in refrigeration and HVAC in England and the United States for 40 years. Previously, he was the U.S. marketing director for Hiross Inc., specializing in computer-room air-conditioning units.

Green Refrigerants

In the evolving world of non-ozone-depleting refrigerants, it is a wise and socially responsible position to specify any of the previously mentioned energy-saving chillers for data centers with a suitable “green” refrigerant. Data centers epitomize the latest technology in industry today. Therefore, it is appropriate that major energy savings are realized whenever possible and that the latest refrigerants are employed in these installations.

The estimations and suggested applications in this article are based on the use of screw compressors with R-407C refrigerant, which is a non-ozone-depleting replacement for R-22. R-407C qualifies for Leadership in Energy and Environmental Design (LEED) credits based on an ozone-depletion potential of 10-5 and a global-warming potential of 1,700. Other common “green” refrigerants include R-134a and R-410A.

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